1. Field of the Invention
This invention relates to a control device and method in a continuously variable transmission system (hereinafter referred to briefly as "CVT") used as a power transmission system in a vehicle, and more particularly to a line pressure control system in the CVT.
2. Description of the Prior Art
In a CVT, an engine power is transmitted by means of a belt racked across a pulley on the input side and a pulley on the output side, and an urging force of the belt by one of the pulleys, normally the pulley on the output side, is controlled by a line pressure of a hydraulic cylinder of the pulley on the output side. If this urging force is small in value, then the belt tends to slip on the pulleys, whereby a torque cannot be transmitted, and, if this urging force is high in value, then heat loss due to the extension or shrinkage of the belt and driving loss of an oil pump are increased, whereby the fuel consumption is increased. In consequecne, it is desired that the line pressure is properly controlled as commensurate to an output torque of an engine or a transmitted torque, however, according to the conventional techniques, the output torque of the engine, which constitutes the base of calculation of the line pressure has been calculated only from an engine rotational speed and an intake throttle opening (Refer to Japanese Utility Model Laid-Open No. 203259/1983 for example). However, the output torque of the engine is variable due to other operation parameters of the engine such as an air-fuel ratio of a gasified fuel-air mixture, and consequently, the fact is that the line pressure is not controlled to a satisfactorily proper value.